The differential geometry of elementary point and line defects in Bravais crystals

Since line defects (dislocations) and point defects (vacancies, self-interstitials, point stacking faults) in Bravais crystals can mutually convert, only theories which comprise these two sorts of defects can be closed in the sense of general field theory. Since the pioneering work of Kondo and of Bilby, Bullough, and Smith it is clear that differential geometry is the appropriate mathematical tool to formulate a field theory of defects in ordered structures. This is done here on the example of the Bravais crystal, where the above-mentioned defects are the only elementary point and line defects. It is shown that point defects can be described by a step-counting procedure which makes it possible to include also point stacking faults as elementary point defects. The results comprise two equations with the appropriate interpretation of the mathematical symbols. The point defects are step-counting defects and are essentially described by a metric tensorg, which supplements the torsion responsible for the dislocations. The proposed theory is meant to form a framework for defect phenomena, in a similar way that Maxwell's theory is a framework for the electromagnetic world.     

Dislocations and internal length measurement in continuized crystals. I. Riemannian material space

Distributions of dislocations creating point defects are considered. These point defects are described by a metric tensor, which supplements a Burgers field responsible for dislocations. The metric tensor depends on the distribution of dislocations and defines a Riemannian geometry of the material space of a continuized crystal and thus an internal length measurement in this crystal. The dependence of the distribution of dislocations on the existence of point defects created by these dislocations is modeled by treating the Burgers field as a field defined on the Riemannian material space. Field equations, following from geometric identities, are formulated as balance equations on this Riemannian space and their source terms, responsible for interactions of dislocations and point defects, are identified.     

On the influence of extrinsic point defects on irradiation-induced point-defect distributions in silicon

A semi-quantitave model describing the influence of interfaces and stress fields on {113}-defect generation in silicon during 1-MeV electron irradiation, is further developed to take into account also the role of extrinsic point defects. It is shown that the observed distribution of {113}-defects in high-flux electron-irradiated silicon and its dependence on irradiation temperature and dopant concentration can be understood by taking into account not only the influence of the surfaces and interfaces as sinks for intrinsic point defects but also the thermal stability of the bulk sinks for intrinsic point defects. In heavily doped silicon the bulk sinks are related with pairing reactions of the dopant atoms with the generated intrinsic point defects or related with enhanced recombination of vacancies and self-interstitials at extrinsic point defects. The obtained theoretical results are correlated with published experimental data on boron-and phosphorus-doped silicon and are illustrated with observations obtained by irradiating cross-section transmission electron microscopy samples of wafers with highly doped surface layers.     

First-principles calculation of point-defective structures of B2-NiSc intermetallics

A pseudo-potential plane-wave method based on first principles was used to calculate the physical parameters of B2-NiSc intermetallics and the geometrical, energetic, and electronic structures of point defects. The possible types of point defects in B2-NiSc intermetallics were analyzed and predicted by comparing the formation enthalpy and formation energy for different kinds of point defect structures. The results show that Ni vacancy defects and Ni anti-position defects are the main point defects in the B2-NiSc intermetallics, and that these point defects emerge as double vacancy defects or double anti-position defects. When the double Ni atoms at Sc sites are in the first nearest neighborhood, the point defect structures are found to be the most stable. An analysis of the electronic structure of NiSc point defects shows that the Ni anti-position defect is more stable than the Sc vacancy defect in the Ni-rich alloy, and the Ni vacancy defect is more stable than the Sc anti-position defect in the Sc-rich alloy. These results are consistent with the energy calculation. Through analysis of C₁₁–C₁₂, G/B and Poisson's ratio, it was found that the Ni_Sc point defect has little effect on crystal plasticity, and that the V_Ni point defect improves crystal plasticity.     

A model study of the electronic structure changes due to impurities with Z ≠ 4, vacancy and carbon at the silicon (111) surface

A tight-binding model for point defects at the silicon (111) surface is developed. It verifies a sum rule and exhibits a great flexibility to study different types of defects. The migration of donors and acceptors to the surface and the role played by surface point defects in surface instabilities are discussed.     

Defect solitons supported by parity-time symmetric defects in superlattices

The existence and stability of defect solitons supported by parity-time (PT) symmetric defects in superlattices are investigated. In the semi-infinite gap, in-phase solitons are found to exist stably for positive defects, zero defects, and negative defects. In the first gap, out-of-phase solitons are stable for positive defects or zero defects, whereas in-phase solitons are stable for negative defects. For both the in-phase and out-of-phase solitons with the positive defect and in-phase solitons with negative defect in the first gap, there exists a cutoff point of the propagation constant below which the defect solitons vanish. The value of the cutoff point depends on the depth of defect and the imaginary parts of the PT symmetric defect potentials. The influence of the imaginary part of the PT symmetric defect potentials on soliton stability is revealed.     

Mechanical properties and defect chemistry

A brief overview of changes in mechanical properties of solids driven by the chemistry of point defects is given. Two major types of effects are considered: direct effects caused by non-interacting point defects and collective effects induced by interacting point defects. The first group includes (1) changes in the linear dimensions of a solid in response to a change in defect concentration and (2) stress induced due to an inhomogeneous distribution of point defects, a so-called chemical stress. The second group includes (1) defect order–disorder transitions accompanied by self-strain and (2) deviations from linear elastic behavior due to the dissociation/association of point defects. All of the above become important if the concentration of point defects is very high (above 10²¹ cm^− 3). These effects may lead to significant anomalies in mechanical properties: spontaneous stress and strain and changes in elastic constants. These effects may significantly affect the application of materials with a large concentration of point defects.     

碳纳米管中点缺陷对热导率影响的正交试验模拟分析

在碳纳米管的制备过程中, 各种点缺陷不可避免地存在于其晶格结构中, 对于碳管的热输运性质造成不可忽视的影响. 使用非平衡分子动力学方法, 选用反应经验键序势能, 模拟计算含有缺陷的碳纳米管的热导率. 尝试采用正交试验方法设计算例, 不但减少了计算量, 并且利于分析缺陷类型、 管长和管径三种结构因素对缺陷造成的热导率下降影响的主次和趋势. 重点研究了掺杂、 吸附和空位三类点缺陷的影响, 与无缺陷完整碳纳米管进行比较, 开展缺陷效应分析, 并进一步考察了环境温度等因素的影响. 模拟结果表明, 相对完整无缺陷碳管, 含有点缺陷的碳管热导率显着下降; 在有缺陷存在的情况下, 缺陷的类型对碳管热导率的影响最大, 管径次之, 管长影响相对最小; 缺陷类型对热导率影响力从大到小依次为: 空位 > 掺杂 > 吸附; 不同环境温度下, 点缺陷对碳管热导率的影响不尽相同.     

可动点缺陷对与运动弯结的滞弹性交互作用

对于在周期分布的运动内应力源作用下,面心立方晶体中Zener对和不对称点缺陷对的微扩散方程采用有限傅氏级数和拉氏变换求解。得出弯结刚性和阻尼系数的有限求和表达式。给出了有关数值计算结果。对于刃型位错与不对称点缺陷对的滞弹性交互作用得出了简单解析结果,与以前用别的方法所得结果相符。讨论了点缺陷内耗的位错增强效应。 关键词：     

CuI晶体及其缺陷态电子结构的模拟

利用相对论密度泛函理论和嵌入分子团簇方法,模拟计算了具有闪锌矿结构的γ态CuI晶体及其缺陷态的电子结构。结果显示晶体的本征能级结构:价带顶主要由I5p和Cu3d轨道杂化组成,导带底由Cu4s轨道组成,禁带宽度为3.1eV,该结果与实验相符。在不同缺陷态的计算中,四面体间隙铜缺陷相对其他间隙缺陷更易于在晶体中形成,其中Cu3d→4s跃迁能量为3.2eV,推测与CuI晶体发光密切相关。     

Statistical mechanics of melting mediated by two types of defects

We propose a defect-mediated melting theory based on the statistics of two types of lattice defects, the point defects and dislocation pairs. The model predicts a first-order phase transition. Based on the model, phase transition temperature, latent heat and other thermodynamic functions are derived. Melting occurs due to discontinuous growth of point defects into dislocation pairs. The calculated phase transition temperature for five alkali metallic crystals are in fair agreement with measured melting temperatures, and the Richards' rule is derived by the model also.     

First-principles study of stability of point defects and their effects on electronic properties of GaAs/AlGaAs superlattice

When the GaAs/AlGaAs superlattice-based devices are used under irradiation environments, point defects may be created and ultimately deteriorate their electronic and transport properties. Thus, understanding the properties of point defects like vacancies and interstitials is essential for the successful application of semiconductor materials. In the present study, first-principles calculations are carried out to explore the stability of point defects in GaAs/Al_0.5Ga_0.5As superlattice and their effects on electronic properties. The results show that the interstitial defects and Frenkel pair defects are relatively difficult to form, while the antisite defects are favorably created generally. Besides, the existence of point defects generally modifies the electronic structure of GaAs/Al_0.5Ga_0.5As superlattice significantly, and most of the defective SL structures possess metallic characteristics. Considering the stability of point defects and carrier mobility of defective states, we propose an effective strategy that Al_As, Ga_As, and Al_Ga antisite defects are introduced to improve the hole or electron mobility of GaAs/Al_0.5Ga_0.5As superlattice. The obtained results will contribute to the understanding of the radiation damage effects of the GaAs/AlGaAs superlattice, and provide a guidance for designing highly stable and durable semiconductor superlattice-based electronics and optoelectronics for extreme environment applications.     

The influence of defects in compound single crystals on the critical angle of planar channeling

The theoretical treatment of the relation between the critical angle of planar channeling and the characteristics of crystal lattice defects is carried out. The predictions are made about some typical forms of the critical angle dependence on the mean-square static displacement produced by defects, and then these predictions are detailed for the cases of homogeneous disordering, spherical clusters of point defects and dislocation loops. Analytical results are supported by the exact computer calculations for the defects in the intermetallic A-15 compounds.     

Temperature dependence of the point defect properties of GaN thin films studied by terahertz time-domain spectroscopy

The dielectric functions of GaN for the temperature and frequency ranges of 10-300 K and 0.3-1 THz are obtained using terahertz time-domain spectroscopy. It is found that there are oscillations of the dielectric functions at various temperatures. Physically, the oscillation behavior is attributed to the resonance states of the point defects in the material. Furthermore, the dielectric functions are well fitted by the combination of the simple Drude model together with the classical damped oscillator model. According to the values of the fitting parameters, the concentration and electron lifetime of the point defects for various temperatures are determined, and the temperature dependences of them are in accordance with the previously reported result. Therefore, terahertz time-domain spectroscopy can be considered as a promising technique for investigating the relevant characteristics of the point defects in semiconductor materials.     

A mesoscopic approach to point-defect clustering in solids during irradiation

Accumulation of point defects in solids during irradiation is often accompanied by self-organization processes which lead to point-defect clustering and thus to the formation of a spatially inhomogeneous defect structure. Within the framework of a mesoscopic phenomenological approach, the conditions for clustering of mobile point defects caused by their elastic interactions are studied. It is shown that differences between the elastic interaction of similar and that of dissimilar defects may lead to such clustering. Further, it is shown that the presence of impurities acting as traps for interstitials may promote the clustering process. The conditions for spatial clustering are studied for characteristic material parameters in order to predict experimental observations of this phenomenon in metals and ionic solids.     

高能电子照射对金刚石中缺陷电荷状态的影响

金刚石经电子辐照后会形成大量的点缺陷, 而这些缺陷很多都是带有电荷的. 提出一种用于研究缺陷电荷状态的新思路, 即利用扫描电子显微镜(SEM)的高能电子照射辐照区域, 通过比较SEM 照射前后的低温光致发光光谱, 为缺陷电荷状态的确定提供了一些依据. 关键词： 金刚石 点缺陷 电荷状态     

Force between a magnetic tip and an inhomogeneous superconductor at zero field

The force exerted by a semi-infinite inhomogeneous superconductor with a planar interface to vacuum on a magnetic tip is studied theoretically in the absence of external magnetic fields. It is shown that the force has a contribution from inhomogeneities due to material defects with unique characteristics. Defects are taken into account in the London limit by allowing the mass parameter to vary spatially. The contribution from defects to the force is calculated analytically to first order in the deviation of the mass parameter from its constant value for the homogeneous superconductor, assuming that the tip is a point dipole perpendicular to the interface, and that it does not spontaneously create vortex matter. Random point defects and linear localized defects are considered phenomenologically. For each defect type the force dependence on the dipole position coordinates is obtained, and the force magnitudes are estimated numerically. The predictions for the dependence of the linear defect force on the dipole lateral position are found to agree qualitatively with experiment.     

Mechanisms of dislocation generation in Si structures with strained layers: Intrinsic point defects in dislocation nucleation

The peculiarities of dislocation production in silicon compositions with elastically strained layers fabricated by the molecular-beam epitaxy technique (SiGe/Si heterostructures) and by direct bonding of Si(110)/Si(100) wafers are studied with transmission electron microscopy. The role of intrinsic point defects during the process of nucleation of misfit dislocations is explained. The surplus concentration of these defects in heterostructures was produced via low-temperature epitaxial growth of buffer layers or with ion implantation of elastically strained heterostructures. The model of “optimal” and “inverse” intrinsic point defects providing an explanation for the relaxation of misfit strains in heterostructures is proposed.     

Radiation-induced modifications of point defects in quartz crystals and their application in radiation dosimetry

Different types of impurity-related point defects in crystalline quartz are known to exhibit various spectroscopic signals. When exposed to ionizing radiations, these defect centers get modified and new species of point defects are formed. This paper presents a study and discussion of radiation-induced modification of point defects in natural as well as cultured quartz, with an emphasis to use this material for radiation dosimetry up to a range of a few Mrad.